1. Field of the Invention
The present invention relates to an MRI auto power control method and system for selecting the optimum power of excitation radio-frequency pulses.
2. Description of the Related Art
The magnetic resonance is a phenomenon in which atomic nuclei having non-zero spins and magnetic moments and placed in a static magnetic field absorb and radiate electromagnetic energy at specific (resonant) frequencies. The atomic nuclei are resonant at an angular frequency .omega.o (=2.pi..nu.o, .nu. is the Larmor frequency) given by EQU .omega.o=.gamma.Ho
where .gamma. is the gyromagnetic ratio inherent in each type of nucleus and Ho is the strength of the applied static magnetic field.
Apparatus for making in vivo diagnosis utilizing the above nuclear magnetic resonance phenomenon processes electromagnetic signals of the same frequency as above induced after resonance absorption to obtain diagnostic information, on a noninvasive basis, in which magnetic resonance parameters, such as atomic nuclear density, longitudinal relaxation time T1, transverse relaxation time T2, flow, chemical shift, etc., are reflected, for example, a cross-sectional image of a selected slice of a human body under examination.
In principle, magnetic resonance diagnostic information can be acquired from the whole body of a patient placed in a static magnetic field. However, constraints on the configuration of apparatus and clinical requirements for diagnostic images allow actual apparatus to excite a selected body portion of a patient and acquire diagnostic information therefrom.
In general, a slice of a body having a certain thickness is selected to be a candidate for imaging. A cross-sectional magnetic resonance (MR) image of the selected slice is produced by performing a data encoding process many times to acquire MR signals, such as echo signals or FID (free induction decay) signals, from the slice and then performing an image reconstruction process based on, for example, two-dimensional Fourier transformation on the MR signals.
Some conventional magnetic resonance imaging (MRI) apparatuses are equipped with an automatic power control (APC) system. The APC is adapted to automatically set the strength of excitation RF pulses at an optimum value. This type of APC has been described and disclosed in Japanese Unexamined Patent Publication No. 61-191946 (laid open on Aug. 26, 1986), U.S. Pat. Nos. 4,806,867 and 4,675,608.
The prior art APC system will be described with reference to FIG. 1. That is, RF pulses (excitation radio-frequency pulses), having their strength varied in steps by varying the attenuation factor of an attenuator, and a slice-selection gradient magnetic field Gs are applied to a slice in a region of interest at preselected pulse repetition intervals, thereby acquiring echo signals from the slice as an MR signal. The RF pulse strength when the maximum MR signal is obtained is identified as the optimum RF pulse strength for that slice. The RF pulse strength determines the flip angles of spins which have a great effect upon image quality. The slice is scanned with RF pulses having the optimum strength. This technique is based on the fact that optimum RF pulse strength varies with regions of a human body and permits optimum MR diagnostic information to be obtained from any region of a human body.
However, a magnetic resonance imaging apparatus equipped with such a conventional APC means has the following problem. That is, in the process of MR signal acquisition with the RF pulse strength varied, in order to, after acquisition of an MR signal by the use of an RF pulse of a certain strength, acquire another MR signal by the use of another RF pulse of a different strength, it is necessary to wait for the longitudinal relaxation of spins once excited. This makes it impossible to shorten the signal acquisition cycle (pulse repetition interval) extremely. The pulse repetition time is generally one second or more. Even if a curve fitting technique is used to decrease the number of measured points, several seconds to tens of seconds will be required to finish all of processes.